1. Field of the Invention
The present invention relates generally to the field of medicine, and more particularly to a method and apparatus for monitoring the metabolic rate of a patient intubated on a ventilator.
2. Description of the Related Art
There are many medical conditions which require that a patient receive assistance in breathing. Such breathing assistance may be provided, for example, by an electromechanical ventilator. A model 7200a ventilator manufactured by Puritan-Bennett Corporation, the assignee of the present patent application, is representative of such devices. The ventilator includes an inspiration conduit that carries inspiration gases such as air or oxygen to the patient and an expiration conduit that carries expired gases from the patient back to the ventilator. These two conduits are typically connected to a "wye" fitting which forms a part of a patient airway adapted to facilitate the flow of the various gases to and from the lungs of the patient through a trachea tube.
The ventilator is adapted to provide certain respiratory information such as inspired and expired flow rates and pressures. However, other information not provided by the ventilator is sometimes required for various medical purposes. In particular, this required information includes the rates at which the patient consumes oxygen and eliminates carbon dioxide, preferably on a breath-by-breath basis.
It has been proposed that breath-by-breath oxygen and carbon dioxide rates be obtained from measurements of the concentration of these gases in the patient airway. According to this approach, a sample conduit would be connected to the wye. A sample of the gases in the wye would be carried through the sample conduit to a set of sensors for measurement of the concentrations of oxygen and carbon dioxide. A flowmeter in the sample conduit would provide flow data which would be used to synchronize the measured concentrations with the flow of the gases in the airway. Flow rate and pressure information from the ventilator would be used to compute inspired and expired flow rates and volumes. All of this information would be corrected for temperature and humidity and would then be used to calculate the required oxygen consumption and carbon dioxide elimination rates.
This proposal has been found to require highly complex mathematical algorithms, particularly in correcting for compliance flow in the conduits. Most devices (ventilators) measure flow at a distance from the patient. Measuring metabolics at the wye either requires an additional flowmeter located at the wye or an extrapolation of flow by distant transducers requiring eliminating compliance flow. Moreover, synchronization of the measured concentrations with the flow is required due to the length of time required for a given sample of gas to flow through the sample conduit from the wye to the sensors, but this sychronization is subject to errors due to patient pressure fluctuations, condensation of H.sub.2 O in the line, obstruction of the line by mucous, kinks or even leaks in the line. The error margin for this synchronization is small because the concentration profiles make a rapid transistion from inspiration to expiration causing a step change which when integrated will cause substantial error if misaligned.
Finally, the response time of the sensors is so critical that a relatively slow response from a sensor can substantially affect the measurement. These problems have made it impractical to implement the breath-by-breath determination of oxygen and carbon dioxide rates by sampling the gases in the wye.
It is possible to determine average oxygen and carbon dioxide rates by means of a mixing chamber in which exhaled gas is captured and held over several breaths. Two sample lines are required--one connected to the wye for measurement of concentrations in the inspiration gases and for waveform analysis, and one connected downstream from the mixing chamber for measurement of concentrations of expired gases. A solenoid valve is used to switch the sensors between the two sample lines. Although this system provides oxygen and carbon dioxide rates, it cannot do so on a breath-by-breath basis. In addition, the mixing chamber is physically bulky and can be difficult to sterlize.
A mixing chamber system is non-functional if the concentration profile exiting the chamber is anything but flat (constant concentration). Therefore, these systems are limited by ventilation rates that "wash-out" the mixing chamber. Continuous flow ventilation is one application in which mixing chamber systems are normally ineffective. A large mixing chamber would be able to handle continuous flow and rapid ventilation, however would also be very insensitive to actual patient changes, thus decreasing its effectiveness.
It will be apparent from the foregoing that there is a need for a way to accurately determine breath-by-breath oxygen consumption and carbon dioxide production rates of a patient on a ventilator.